The CMS should be approx. in the center of the measuring electrodes, for example on the nose or the forehead.

The DRL can be anywhere on the body. In theory, a location at some distance of the measuring sites is optimal. In practice, a location not too far from the CMS often results in lower interference levels (I don't have a good explanation). Some trial and error to archive the best location for minimal interference levels (in the unreferenced display) may be useful.